We need the GNU version of dd which is provided by coreutils:
brew install coreutils
Then locate the SD card using diskutil:
diskutil list
It will probably be something like /dev/disk4.
Copy the SD card image onto local drive:
sudo gdd if=/dev/disk4 of=sdcard.img bs=32M status=progress
To copy the image back onto an SD card, insert the SD card and locate the disk with diskutil as before.
Unmount the SD card:
sudo diskutil unmountDisk /dev/disk4
Then copy the image onto the SD card using dd:
sudo gdd if=sdcard.img of=/dev/disk4 bs=32M status=progress